In optical communications systems, it is necessary to characterize the phase and amplitude of optical pulses as accurately as possible in order to predict and mitigate signal degradation. For example, in long distance wavelength-division multiplexed (WDM) systems, optical signals are modified by linear effects such as chromatic dispersion and polarization-mode dispersion, and nonlinear effects such as self-phase modulation or cross-phase modulation, all of which degrade their transmission properties. Characterizations of the effect of the distortions on a propagating optical signal can assist in determining corrective measures for an optical communications system. Another example of the necessity of having appropriate temporal diagnostics lies in the wide variety of modulation formats that are used for data transmission. Information bits are encoded in the amplitude or phase of short optical pulses, either in a binary fashion or with multiple levels of coding.
The temporal characterization of optical signals may be achieved by various means depending on the properties of the optical signal and the application. For example, one might be interested in measuring the electric field (temporal intensity and phase, or equivalently spectral intensity and phase) of a periodic train of pulses, from which information such as the duration or the chirp of the pulse (and more generally, the properties of the medium in which the pulse has traveled) may be deduced. In digital optical telecommunications where information is transmitted using optical pulses, a well-adapted characterization tool is the eye diagram, which consists of samples of the temporal intensity of a signal under test. A synchronous eye diagram therefore offers a statistical representation of allowable values for the temporal intensity of an optical signal. This information is extremely valuable in on-off keying systems since it represents the temporal shape of the pulses carrying the bits of information in the data stream and may be used to track various sources of noise and impairments. However, a conventional eye diagram only contains intensity information, and methods to measure such a representation do not have the ability to get any phase information. This is particularly detrimental for transmission systems based on phase-encoding, such as differential phase shift keying (DPSK), where the information is typically encoded in the phase difference between pulses in the data stream that have identical temporal intensity. The use of such phase encoding advantageously offers a relatively high degree of noise immunity while facilitating comparatively high data rates and is becoming common in telecommunication systems. As such, what is needed in the art is a method and apparatus for the direct characterization of the phase of phase-encoded signals.